1. Field of the Invention
The present invention relates to a semiconductor device comprising a thin film transistor (hereinafter referred to simply as a "TFT"), an to a process for fabricating the same.
2. Prior Art
Thin film transistors have been applied heretofore to active-matrix addressed liquid crystal display devices or to image sensors. In particular, the active matrix liquid crystal display devices using TFTs as switching elements, i.e., those comprising TFTs being formed in each of the pixels, are attracting much attention as liquid crystal display devices capable of operating at high speed.
As is well known, light must be transmitted through the pixel portions of a liquid crystal display device. However, the electric conductivity of the active layer of a TFT increases as light is irradiated thereto, because the active layers are made of a film of amorphous silicon or crystalline silicon which is usually photosensitive. The increase in conductivity of an active layer then unfavorably impairs the charge retention of a pixel electrode, because the off current, which is an important characteristic of a TFT switching element, is increased by the irradiation of light. Accordingly, it is necessary to establish a special constitution for the TFT, i.e., a constitution in which light is not irradiated to the active layer, and particularly, in which the channel-forming region of the active layer is isolated from light.